The present invention relates to a method for hermetically sealing a high output semiconductor laser element used for an optical fiber amplifier, etc., and a hermetic-sealing apparatus for semiconductor laser elements in the field of optical transmissions.
Conventionally, a semiconductor laser module has been frequently utilized as a signal light source and a pumping light source for an optical fiber amplifier in the field of optical transmissions.
In particular, those having wavelength bands of 980 nm, 1020 nm and 1480 nm have been known as a light source module for pumping rare metal doped optical fibers in optical fiber amplifiers. There are many cases where an optical output of more than 100 mW is required with respect to the use of such a light source.
FIG. 1 shows an example of the structure of such a high output semiconductor laser module. In FIG. 1, light emitted from a semiconductor laser element 11 is optically coupled with an optical fiber 12 disposed in proximity of the front end face 11a of the semiconductor laser element 11, and is led to the exterior, where the light is used for an appointed application. The semiconductor laser element 11 is fixed on a thermomodule 14 for adjusting a temperature, and these devices 11 and 14 are hermetically enclosed in a package 13.
In order to produce such high output semiconductor laser modules 10, first, the lower plate 14a of the thermomodule 14 is fixed by soldering on the bottom plate 13b of a package 13. At the thermomodule 14, a lead line 14c is soldered to a lead terminal (not illustrated) of the package 13 for electrical connection with the exterior of the package 13.
Next, a substrate 15 having a fixed semiconductor laser element 11 is soldered to and fixed on the upper plate 14b of the thermomodule 14. An optical fiber 12 is inserted through a through hole 13a provided at the sidewall of the package 13 and is introduced into the interior of the package 13, and is positioned so that most of light from the semiconductor laser element 11 is coupled thereto. And, the optical fiber 12 is fixed on the substrate 15 by YAG laser welding in the positioned state. In addition, the optical fiber 12 is also soldered at the through hole 13a. 
After that, a cover 13c is placed on the upper surface of the package 13, and the circumferential portion thereof is welded by resistance welding to be hermetically sealed from the exterior.
A degradation mechanism of a high output semiconductor laser module is a PIF (Packaging Induced Failure).
The PIF is produced by a slight amount of hydrocarbons in the module being subjected to a polymerization reaction by a photochemical reaction and being adhered to the light emitting end face of a semiconductor laser element as solid organic substances. That is, the adhered organic substances absorb a laser light, thereby causing the temperature at the end face of the semiconductor laser element to rise, whereby the end face of the semiconductor laser element is melted and broken.
Herein, hydrocarbons may be ah organic solvent used for cleaning in a production process of semiconductor laser modules, flux used for soldering, etc. Even though a slight amount of hydrocarbons should remain in a package at a ppm level or less, they will become a cause of such a degradation.
The phenomenon results from a photochemical reaction made by a laser light, especially in cases where the optical output from a semiconductor laser element is large, and the laser light has a great optical energy with a very short wavelength. This is a failure mode which remarkably occurs, for example, in a case of 980 nm and 1020 nm bands which emit an optical output of 100 mW or more. A detailed report regarding the failure has been published, for example, xe2x80x9cRequirements to avert packaging induced failure of high power 980 nm laser diodesxe2x80x9d, Jackobson et al, LEOS, November 1993.
In order to prevent a failure of a semiconductor laser element by the PIF, a method for containing oxygen in a hermetically sealed gas 16 of the package 13 has been employed previously.
That is, since polymerization of a slight amount of hydrocarbons in the package can be suppressed due to the existence of oxygen, or by oxidation of the adhered organic substances, the organic substances can be prevented from being accumulated on the end face of the semiconductor laser element.
However, in a prior art high output semiconductor laser module, which is sealed with a sealing gas containing oxygen, although it is possible to prevent organic substances from being adhered to and being accumulated on the light emitting end face of a semiconductor laser element by actions of the oxygen sealed within, water is generated by a reaction of oxygen and the residual hydrocarbons or reaction of oxygen and organic substances adhered to the light-emitting end face, whereby the water content in the package is increased. The water generated is condensed to dew at the light-emitting end face to hinder optical coupling, whereby the optical output is lowered or short-circuits result therefrom in the interior electric wiring.
As a principle, the above phenomenon can be avoided by reducing the amount of the residual organic substances in the package. But, PIF may occur due to the existence of a very slight amount of organic substances at a level of ppm or less. In a fact, it was difficult to repeatedly achieve a very low level of organic substance which does not produce any PIF.
The present invention was developed to propose a method for maintaining the cleanliness of packages in a stabilized state and in a good condition in order to prevent organic substances from being adhered to the end face of the abovementioned high output semiconductor laser element due to photochemical reactions.
That is, a method for hermetically sealing a semiconductor laser element according to a first aspect of the invention comprises a first step of introducing oxygen into a chamber of a hermetic-sealing apparatus and irradiating ultraviolet rays to an unsealed package where a semiconductor laser element is mounted in the chamber; and a second step for purging the inside of the chamber with an inert gas and hermetically sealing an unsealed package in the inert gas atmosphere after the first step is completed.
A method for hermetically sealing a semiconductor laser element according to a second aspect of the invention is featured in that, in the second step of the method for hermetically sealing a semiconductor laser element according to the first aspect, ultraviolet rays are irradiated onto the purging inert gas, and an unsealed package is hermetically sealed in the inert gas atmosphere onto which the ultraviolet rays are irradiated.
A method for hermetically sealing a semiconductor laser element according to a third aspect of the invention comprises a first step for introducing oxygen and an inert gas into a chamber of a hermetic-sealing apparatus and irradiating ultraviolet rays onto an unsealed package having a semiconductor laser element mounted in the chamber, and a second step for hermetically sealing the unsealed package in the atmosphere consisting of the oxygen and inert gas without being exposed to any outer atmosphere.
Further, a method for hermetically sealing a semiconductor laser element according to a fourth aspect of the invention comprises a first step for introducing oxygen and an inert gas into a chamber of a hermetic-sealing apparatus and irradiating ultraviolet rays onto an unsealed package having a semiconductor laser element mounted in the chamber; and a second step for purging the inside of the chamber with the inert gas and hermetically sealing the unsealed package in the inert gas atmosphere after the first step is completed.
Further, a method for hermetically sealing a semiconductor laser element according to a fifth aspect of the invention comprises a step of introducing a package having a semiconductor laser element mounted, into a first chamber of a hermetic-sealing apparatus, introducing oxygen into the first chamber, and irradiating ultraviolet rays onto an unsealed package having a semiconductor laser element mounted; a step of transferring an unsealed package having a semiconductor laser element mounted, from the first chamber which is interrupted from the outer atmosphere, to a second chamber in an inert-gas purged state; and a step of hermetically sealing the unsealed package having a semiconductor laser element mounted, in an inert gas atmosphere in the second chamber.
Further, a method for hermetically sealing a semiconductor laser element according to a sixth aspect of the invention comprises a step of introducing a package having a semiconductor laser element mounted, into a first chamber of a hermetic-sealing apparatus, introducing oxygen into the first chamber, and irradiating ultraviolet rays onto an unsealed package having a semiconductor laser element mounted; a step of blending an inert gas and oxygen by a gas blender having an ultraviolet ray irradiating means while irradiating them and introducing the same into a second chamber, a step of transferring an unsealed package having a semiconductor laser element mounted, from the first chamber which is interrupted from the outer atmosphere, to a second chamber; and a step of hermetically sealing the unsealed package having a semiconductor laser element mounted, in a blended gas atmosphere of the inert gas and oxygen introduced from the gas blender in the second chamber.
With a method for hermetically sealing a semiconductor laser element according to the first through the sixth aspects of the invention, a very slight amount of organic substances remaining in a package can be effectively decomposed and removed by irradiation of ultraviolet rays. In particular, since ultraviolet rays are irradiated onto oxygen, and ozone and excited oxygen atoms are generated, whereby these are reacted with the decomposed organic substances, the effect of removing organic substances is remarkable. Furthermore, since not even a slight amount of organic substances and hydrocarbon contained in the atmosphere of a chamber enter a package during the sealing work, hermetic sealing can be carried out in a very clean atmosphere.
Further, with a method for hermetically sealing a semiconductor laser element according to the second and sixth aspects of the invention, even though a slight amount of, for example, organic substances is contained in an inert gas to be purged, the organic substances in the inert gas can be decomposed and removed by irradiation of ultraviolet rays, whereby it becomes possible to hermetically seal unsealed packages in a clean atmosphere not containing any organic substance. In particular, in a method according to the sixth aspect of the invention, since an inert gas and oxygen, which are introduced into the second chamber are both irradiated by ultraviolet rays in a gas blender, organic substances can be decomposed and removed by ultraviolet rays even though a very slight amount thereof is contained in the inert gas. In addition, since ozone and excited oxygen atoms generated from oxygen by irradiation of ultraviolet rays are reacted with the decomposed organic substances, the organic substances which may be contained in the inert gas can be completely removed, whereby unsealed packages can be hermetically sealed in a very clean atmosphere not having any hydrocarbons and organic substances.
Still further, a method for hermetically sealing a semiconductor laser element according to other aspects of the invention is featured in that, where the semiconductor laser element is hermetically sealed by the method according to the first through the sixth aspects of the invention, an unsealed package is rinsed in advance by an organic solvent before introducing the same into a chamber of a hermetic-sealing apparatus.
According to such a hermetic-sealing method, since a majority of decomposed organic substances in a package are removed by rinsing with an organic solvent and the remaining organic substances removed by ultraviolet rays, it is possible to shorten the period of time for removing organic substances by irradiation of ultraviolet rays in the chamber, thereby improving the working efficiency of hermetically sealing semiconductor laser elements.
An apparatus for hermetically sealing a semiconductor laser element according to the first aspect of the invention comprises a chamber in which an unsealed package having a semiconductor laser element mounted is carried, a means for introducing an inert gas into a chamber, a means for introducing oxygen into the chamber, a means for generating ultraviolet rays in the chamber, and a resistance welder for sealing an unsealed package in the chamber.
Also, an apparatus for hermetically sealing a semiconductor laser element according to the second aspect of the invention comprises a first chamber, a second chamber, and a means for openably partitioning the first chamber and the second chamber, wherein the first chamber has an oxygen introducing means and an ultraviolet ray generating means, and the second chamber has an inert gas introducing means and a resistance welder.
In addition, an apparatus for hermetically sealing a semiconductor laser element according to the third aspect of the invention has a gas blender for blending oxygen and an inert gas to be introduced, and irradiating ultraviolet rays onto the blended gases, wherein the second chamber has a means for introducing blended gases, onto which ultraviolet rays are irradiated, from the gas blender.
A hermetic-sealing apparatus according to the first aspect of the invention executes a hermetic-sealing method according to the first and the fourth aspects, and an apparatus for hermetically sealing a semiconductor laser element according to the second aspect can execute a method for hermetically sealing a semiconductor laser element according to the fifth aspect, and further an apparatus according to the third aspect can execute a method for hermetically sealing a semiconductor laser element according to the sixth aspect.
Therefore, with a method and an apparatus for hermetically sealing a semiconductor laser element according to the present invention, it will become possible to carry out hermetic sealing of semiconductor laser elements in a very clean atmosphere almost free from hydrocarbons and remaining organic substances, whereby PIF can be prevented from occurring, and problems in the prior arts, in which moisture is generated, can be prevented, and it becomes possible to produce semiconductor laser elements having high reliability.